Infant mortality and morbidity associated with perinatal brain injury represent significant clinical problems, as both the cost of immediate medical care and long-term support continues to escalate. However, the mechanisms underlying neonatal brain damage are still poorly understood. In one model of brain injury, using the selective antagonist MK801, we have shown that blockade of the N-methyl-D-aspartate receptor (NMDAR) during a critical period in neonatal development will promote widespread injury throughout the forebrain. As the animal matures this window of vulnerability closes and the brain is no longer sensitive to MK801 treatment. Using tissue culture approaches, we have shown that neuronal injury following MK801 exposure is associated with decreased intracellular calcium. In similar preparations, we have demonstrated that agents that reduce intracellular calcium by mechanisms independent of NMDAR blockade also promote neuronal injury. However, what is not known is whether reductions in intracellular calcium during this window of vulnerability (regardless of mechanism) can lead to the same brain injury as MK801 treatment. We have recently shown that calcium binding proteins are not expressed in MK801-sensiitve neurons. In addition, these same neurons appear unable to buffer changes in calcium. Based on our in vitro observations, we hypothesize that MK801 exposure promotes neonatal brain injury by reducing intracellular calcium. We will address this hypothesis by using whole animal, brain slice and transgenic approaches. We will correlate changes in intracellular calcium with changes in markers of programmed cell death, to identify potential pathways. Because many drugs used in neonatal surgery act to lower intracellular calcium, understanding the mechanisms behind the selective toxicity of MK801 in neonates will help identify new therapeutic targets to protect from or cure brain injury in infants exposed to such agents during vulnerable periods. However, if our hypothesis is correct, maintaining calcium homeostasis may be generally important to neuronal survival during critical developmental periods and a loss of calcium homeostasis may be a significant factor contributing to diverse forms of perinatal brain injury.